1. Technical Field of the Invention
This invention broadly relates to the treatment of a subterranean formation containing crude oil with an acidic composition to aid in the recovery of the crude oil from the formation. The invention more specifically relates to the acid treatment of such a formation in the presence of ferric ion and or sulfides. The invention still further relates to the treatment of the formation with a specific aqueous acidic composition formulated to prevent the precipitation of ferric hydroxide, free sulfur and metal sulfides in the formation and to resist the formation of sludge in the crude oil.
This invention still further pertains to the removal of sulfide from acidic solutions and to compositions for and methods of controlling the precipitation of metal sulfides when treating a sour oil and gas well in the presence of ferric ion with an acid solution.
For purposes of this invention, sludge is defined as a solid material formed in crude oil containing asphaltenes and maltenes which constituents may, under certain conditions, precipitate from the crude oil. Formation of sludge in crude oil while the crude oil is in the formation can render very difficult the task of recovery of the oil from the formation. For purposes of this invention crude oil containing quantities of asphaltenes and maltenes subject to the formation of sludge is referred to as heavy crude and sometimes as sludging crude.
2. Description of the Prior Art and Problem Solved
Formation acidizing or, simply, acidizing, is a method, well known in the prior art, utilized to increase the flow of fluid from a subterranean formation. Under these methods the formation is contacted with an acidic composition to react with and dissolve materials contained therein for the purpose of increasing the conductivity of the formation. The flow of fluid from the formation is therefore increased because of the increase in formation conductivity caused by the dissolution of the material.
A known method of acidizing a subterranean formation comprises the steps of: conducting an acid composition to the formation through tubing disposed in a borehole penetrating the subterranean formation; forcing the acid composition into contact with the formation; and permitting the acid to react with and dissolve certain materials contained therein to thereby enlarge pore spaces within the formation and thus to increase the permeability thereof. Acidizing calcareous formations, such as limestone, has been successfully conducted utilizing hydrochloric acid, certain organic acids, such as acetic acid, citric acid and formic acid, and mixtures thereof.
The object of formation acidizing--increasing formation conductivity--can be frustrated if the specific acid employed in the treatment provides an environment which fosters the production of precipitates within the formation which fill and plug the pore spaces in the formation with the consequent result of failing to increase and possibly even of decreasing formation conductivity.
The precipitation of compounds containing iron, nonferrous metals, free sulfur and metal sulfides has been a problem encountered in acidizing operations. One such problem features the precipitation of iron compounds, such as ferric hydroxide, from the acid composition used to perform the acidizing. Hydrochloric acid upon spending to pH values of about 2.5 and greater permits the precipitation of ferric hydroxide.
In one aspect, the acid may dissolve iron-containing scale from pipe and equipment and iron-containing minerals present in the formation. As the acid reacts, with consequent increase in pH of the acid solution, dissolved iron present in the solution in the ferric, Fe (III), oxidation state precipitates as ferric hydroxide (Fe(OH).sub.3). Ferric hydroxide precipitate can plug the formation and thus cause serious well damage. Iron control is thus a problem.
In another aspect, the acid may dissolve ferrous iron-containing scale from pipe and equipment and ferrous iron-containing minerals present in the formation. As the acid spends, with consequent increase in pH of the acid solution, dissolved iron present in the solution in the ferrous, Fe (II), oxidation state, upon contact with sulfide present in the solution, can precipitate as ferrous sulfide. Ferrous sulfide precipitate can plug the formation and thus cause serious well damage. Sulfide control is thus a problem.
Hydrochloric acid may precipitate sludge from the crude oil contacted. The quantity of precipitated sludge is related to formation temperature, acid concentration, ferric iron concentration and the concentration of asphaltenes and maltenes in the crude oil.
In other instances, wells containing quantities of sulfide and particularly hydrogen sulfide are sometimes referred to as "sour wells". In these instances, the combination of sulfide ions and iron create precipitation problems. Sulfide ions reduce ferric ions to ferrous ions by the following reaction: EQU 2Fe.sup.3+ +S.sup.2-.fwdarw.S.sup.o.dwnarw.+2Fe.sup.2+
This reaction is beneficial because ferric ions are reduced to ferrous ions with the result that precipitation of ferric hydroxide is reduced. However, the reaction is detrimental because as a result thereof, elemental or free sulfur is caused to precipitate. Sulfur precipitate can plug a formation and thus cause serious well damage.
In addition, sulfide ions can also react with ferrous ions to form ferrous sulfide, FeS, which also precipitates.
S.sup.2- +Fe.sup.2+.fwdarw.FeS.fwdarw.
It is believed that sulfide ion and ferrous iron in an acid solution form a precipitate when the pH of the acid is about 1.9. The pH value of completely spent acid used to treat subsurface formations is greater than 1.9, therefore, any ferrous sulfide which may be dissolved in the unspent acid solution will precipitate when pH rises to a value of about 1.9. Like free sulfur, ferrous sulfide precipitate can plug a formation and cause serious damage.
This invention thus addresses problems encountered in acid treatment operations conducted in the presence of ferric ions, free sulfur, and/or sulfides. Therefore, when treating a sour well with acid contaminated with iron, a problem to be solved is to prevent or at least to reduce the precipitation of ferric hydroxide, free sulfur and ferrous sulfide.
Attempts to control precipitation of ferrous sulfide by maintaining the pH of the acid solution below 1.9 have failed, because it is not feasible, in most acid treatment operations, to maintain the pH of the acid solution below 1.9 during the entire operation.